This invention relates to herbicidal compositions and methods of use. In particular, this invention relates to herbicidal compositions comprising an herbicidally effective amount of a thiolcarbamate herbicide in combination with an extending amount of an extender compound. An extender compound is a certain thiolcarbamate, thiolcarbamate sulfone, thiolcarbamate sulfoxide, thionocarbamate, dithiocarbamate or carbamate compound disclosed herein. An extender serves to prolong the effectiveness of single or multiple applications of the thiolcarbamate herbicide in controlling undesired plant growth.
Thiolcarbamates are well known in the agricultural art as herbicides useful for weed control in crops such as corn, potatoes, beans, beets, spinach, tobacco, tomatoes, alfalfa, rice, peanuts and soybeans among others. Thiolcarbamates are primarily used in pre-emergence application. They have been found to be particularly effective when incorporated into the soil prior to the planting of the crop. As a herbicide, the thiolcarbamate is most concentrated immediately after its application. How long thereafter the initial concentration is retained depends in large part on the particular soil used. Thus the rate at which the thiolcarbamate herbicide concentration declines following its application tends to vary from one type of soil to the next. This is evident in the observable extent of actual weed control after considerable time has elapsed.
The extender compounds of the instant invention extend the soil life of thiolcarbamate herbicides in soil which has been treated one or more times before with a thiolcarbamate herbicide or in soil which has had no previous treatment with a thiolcarbamate herbicide. Soil which has been previously treated with a thiolcarbamate herbicide is termed "pretreated soil"; whereas soil which has not been so treated is termed "non-pretreated soil."
Under certain environmental and other conditions, thiolcarbamate herbicides are degraded more rapidly in pretreated soil of certain types than in non-pretreated soil. Such pretreated soil is considered conditioned to degrade thiolcarbamates herbicides at an accelerated rate compared to non-pretreated soil.
The extenders of the instant invention prevent rapid soil degradation of thiolcarbamate herbicide. Not only do the extenders prevent the rapid soil degradation of thiolcarbamate herbicides in pretreated soil, but also prolong the herbicidally effective soil life of thiolcarbamate herbicides in non-pretreated soil.
The soil in many of the examples below showing activity of representative extenders of the instant invention has been pretreated. Such pretreatment is designed to simulate field conditions wherein a field has been repeatedly treated with a thiolcarbamate herbicide. This simulation is effected by retreating the experimental soil, employed herein within weeks of the first treatment rather than seasons later as in the fields. Such a short experimental retreatment period highly conditions the soil.
The improvement in the soil persistence of the thiolcarbamate herbicide effected by the extender compound can manifest itself in a variety of ways. Improved soil persistence can be shown by herbicidal efficacy tests, wherein the degree of weed injury is measured after a set period of time following application of the herbicide. In such a test, the extender compound is shown to increase the herbicidal effectiveness of the thiolcarbamate by increasing the persistence of the latter in the soil, and thus prolonging its effective life.
Several of the thiolcarbamate extenders within the scope of the instant invention are specifically disclosed as phytocides in Great Britain Patent Specification No. 862,548 (Tilles et al., published March 1961) and as herbicides in U.S. Pat. Nos. 2,989,393 and 2,916,370. However, in all three of these references the rates in lb/acre at which the compounds were applied are very high compared to commercial application rates and those employed in the examples herein. Similarly, thiolcarbamates related to the extenders of the instant invention are disclosed as herbicides when applied at very high rates in U.S. Pat. Nos. 2,901,498 and 2,916,369. Nowhere in any of the above-cited patent references is the problem of rapid soil degradation of thiolcarbamate herbicides even mentioned nor is the extension of soil life of thiolcarbamates referred to anywhere therein.
For a thiolcarbamate, thiolcarbamate sulfone, thiolcarbamate sulfoxide, thionocarbamate, dithiocarbamate or carbamate compound to be considered an extender of this invention, it is highly desirable for there to be a significant difference between the herbicidal activity of the extender when tested alone and the herbicidal activity of the thiolcarbamate and extender composition, when applied at the same rate and by the same method.